1. Field of Invention
The present invention relates to an MIMO (Multiple Input Multiple Output) OFDM (orthogonal frequency division multiplexing) system, in particular to a signal processing method and device in an MIMO multi-carrier system.
2. Description of Prior Art
In a multi-antenna MIMO wireless communication system, the receiving end and the transmitting end can support a plurality of independent wireless channels with the same bandwidth in fully scattered environment. Therefore, system capacity can be effectively improved, which is useful in settling the problem of capacity bottleneck for future wireless communications. In many real-world applications, as shown in FIG. 1, channel parameters can be obtained over a low rate feedback channel between the receiving end and the transmitting end. With this feedback information, pre-coding can be implemented on transmitted data in the transmitter, so that a greater gain can be obtained in the receiving end to improve the entire system's reliability and the communication distance.
FIG. 2 illustrates a specific MIMO solution implemented in an orthogonal frequency division multiplexing (OFDM) system. The transmitter 102 uses multiple antennas to transmit a plurality of data streams to one or more receiving devices 101 by a plurality of OFDM sub-carriers. S(K) denotes the plurality of data streams, where K(1≦K≦N) represents the sub-carrier index, N denotes the number of sub-carriers, Ns indicates the number of data streams on each sub-carrier, and Ns≧1. For example, data streams on sub-carrier 1 can be denoted as:S(1)=[S1(k=1),S2(k=1), . . . , SNs(k=1)]
The data streams on each sub-carrier is mapped to a new set of data streams X(K) through the pre-coding generator 105:X(K)=V(K)ST(K),X(K)={X1(k=K),X2(k=K), . . . , XTx(k=K)}T 
X(K) are fed into Tx transmitting antennas 104, and each signal is generated through IDFT (inverse discrete Fourier transform) 107, cyclic prefix insertion and serial-to-parallel converter 106. In FIG. 2, V(K) denotes a precoding matric for subcarrier K.
FIG. 3 shows pre-coding symbol generators 105 in greater detail, which function before each sub-carrier input of the Tx IDFTs 107 in the transmitting device 102. As shown in FIG. 3, an MIMO pre-coding matrix represented by a Tx×Ns matrix V(K)={v1(K),v2(K), . . . , vNs(K)} is used by each of the pre-coding symbol generators 105 in the transmitting device 102 for weighting. Here, Tx indicates the number of transmitting antennas. To obtain the value of V(K), the transmitting device 102 needs the response information on channel(s) between the transmitting antennas 104 and one or more receiving antennas in the receiving device 101.
As can be seen from FIGS. 1-3, the transmitter 102 of the system needs to know the matrix frequency response between the transmitting and the receiving arrays, which gives rise to trouble in the case of fast varying frequency selection broadband channels, such as the type of channels in an OFDM (OFDMA) mobile communication system. In more detail, in the feedback-based MIMO system illustrated in FIG. 1, it is necessary to feed the pre-coding weighting matrix back to the transmitter 102 and update it, in order to keep track of channel variations spanning over time and frequency for optimal performance. However, mechanisms allowing complete track of channel response may require feedback of forbidden level between the receiving device and the transmitting device. Therefore, a method and device for MIMO transmission in a communication system is required, in which no feedback of forbidden level is required when channel information to be used by the transmitting device is sent back to the transmitting device.
At present, the problem of feedback in an MIMO OFDM/OFDMA system is a hot spot in research by such standardization organizations as the 3GPP LTE, the WiMAX, the WiBro and so on. There is no definite and ultimate solution in current protocols or specifications.